Thermo-mechanical Process for Martensitic Bearing Steels

ABSTRACT

A method for treating a steel bearing component includes subjecting the steel bearing component to a scouring treatment to impart compressive residual stress to a surface region thereof and subsequently nitriding the steel bearing component  1.  The resulting steel bearing component exhibits a nitrided case depth of about 0.002-0.014 inches and a compressive residual stress value greater than −120 ksi is achieved at a depth of 0.002 inches and −20 ksi to a minimum depth of 0.010″. The steel bearing component may be a race and/or a rolling element, such as a ball.

CROSS-REFERENCE

This application claims priority to U.S. provisional application No. 61/747,836, filed on Dec. 31, 2012, the contents of which are incorporated hereby by reference in their entirety.

TECHNICAL FIELD

The present teachings generally relate to improved processes for manufacturing bearing components for roller (rolling element) bearing applications, as well as to improved bearing components and roller (rolling element) bearings containing the same.

BACKGROUND ART

High performance mechanical systems such as bearings and gears in advanced gas turbine engines are required to operate at ever increasing speeds, temperatures and loads. Known bearing steels have been developed to handle the existing operating conditions but are limited in achieving higher speeds and loads under current heat treatment and processing operations.

Processing operations, both mechanical and thermal, have been developed to improve surface hardness and residual stress in order to affect bearing life. Operations such as scouring and peening are mechanical operations designed to impart compressive stress into the bearing component. Thermal processing such as nitriding has been developed to increase the component's ability to improve surface residual stress and hardness, providing both increased fatigue life and a better ability to handle debris damage. See e.g., U.S. Pat. No. 6,966,954.

Additional references that provide background for the present teachings are:

-   1. C. M. R. Wilkinson and A. V. Oliver, “The Durability of Gear and     Disc Specimens—Part I: The Effect of Some Novel Materials and     Surface Treatments”, Tribology Transactions, Vol. 42, 1999, pp.     503-510; -   2. David P. Davies, “Duplex Hardening: An Advanced Surface     Treatment”, Heat Treating, August 1992 -   3. N. Binot, A. Viville, H. Carrerot, “Duplex treatment, carburizing     and nitriding of a secondary hardening steel, Application to     aeronautical ball bearings”, Treatments and Tribology, October     2001—No. 334

However, there is still a long-felt need to provide superior fatigue life characteristics in bearing components.

SUMMARY

The present teachings achieve surprisingly improved fatigue life characteristics in steel components, e.g., for bearing applications, by subjecting a steel bearing component (such as bearing balls) to both scouring and nitriding processes, which increases the magnitude of the compressive residual stress in the steel component, thereby improving the ability of the steel component to handle higher loads and debris damage without premature failure. An increase in rolling contact fatigue life is also a benefit of this combination of processes.

BRIEF DESCRIPTION OF THE FIGURE

The sole FIGURE shows a comparison of compressive residual stress profiles for bearing balls prepared according to the present teachings (Scoured/Nitrided Balls) and bearing balls prepared without both scouring and nitriding.

DETAILED DESCRIPTION OF THE INVENTION

Bearings are basically comprised of an inner race (ring), an outer race (ring) and roller (rolling) elements (e.g., balls) disposed therebetween. In preferred aspects of the present teachings, one or more components of a roller (rolling element) bearing are made from steel that has been scoured (peened) and then nitrided (case-hardened) to a depth of about 0.002-0.014 inches (2 to 14 mils), preferably about 0.008 inches (0.20 mm). Preferably, the steel is M50 steel, M50NiL steel or 52100 steel.

The nitriding treatment preferably avoids the formation of intergranular precipitates and also does not result in the decarburization of the surface of the steel.

While the entire surface of one or more of the bearing elements may be nitrided, portions of the bearing surfaces may be masked, if desired so as not to be nitrided. Preferably, at least the roller (rolling) elements have been treated according to the present teachings, but it is also possible to treat other wear surfaces according to the present teachings, such as the raceway surfaces of the inner race and the outer race as well as the lands of the raceways. The bearing may include a cage, separator or retainer for separating the roller (rolling) elements, which may be comprised of any suitable material.

M50 steel is often used as a bearing material for aircraft engine applications and is comprised of, in weight percent, about 0.80-0.85% carbon, about 4.00-4.25% chromium, about 4.00-4.50% molybdenum, about 0.15-0.35% manganese, about 0.10-0.25% silicon, about 0.9-1.10% vanadium, 0.015% max. phosphorus, 0.010% max. sulfur, 0.15% max. nickel, 0.25% max. cobalt, 0.25% max. tungsten, 0.10% max. copper and the balance being essentially iron.

M50NiL steel is a low carbon, high nickel variant of the M50 alloy. M50NiL also has been often used as a bearing material for aircraft engine applications, and is comprised of, in weight percent, about 0.11-0.15% carbon, about 4.00-4.25% chromium, about 4.00-4.50% molybdenum, about 0.15-0.35% manganese, about 0.10-0.25% silicon, about 3.20-3.60% nickel, about 1.13-1.33% vanadium, 0.015% max. phosphorus, 0.010% max. sulfur, 0.25% max cobalt, 0.25% max. tungsten, 0.10% max. copper and the balance being essentially iron.

52100 steel is a high carbon, low alloy steel. 52100 also has been often used as a bearing material for aircraft engine applications, and is comprised of, in weight percent, about 0.93-1.05% carbon, about 1.35-1.60% chromium, about 0.010% max molybdenum, about 0.25-0.45% manganese, about 0.15-0.35% silicon, about 0.25% max nickel, about 0.025% max. phosphorus, 0.015% max. sulfur, 0.050% max aluminum, 0.0015% max. oxygen, 0.30% max. copper and the balance being essentially iron.

As used herein, the expression “the balance being essentially iron” is understood to include, in addition to iron, e.g., small amounts of impurities and/or other incidental elements, some of which have been described above, that are inherent in steels, which in character and/or amount do not affect the advantageous aspects of the steel.

Prior to the below-described scouring and nitriding process steps, rolling elements, such as balls, may be prepared, in general, by forming ball blanks(heading the ball blanks), soft grinding, heat treatment and then hard grinding. Standard manufacturing techniques known in the art may be utilized for all of these steps.

The above-noted steels have particular advantageous properties when prepared as martensitic steels. That is, as bearing materials, beneficial properties are achieved by forming a fully martensitic structure and by avoiding or minimizing retained austenite. Retained austenite is soft and may adversely affect the properties of the M50, M50NiL or 52100 steel bearing material and therefore, it is best to be minimized, e.g., by using heat treatment techniques well known in the art such as rapid and severe quenches in suitable quench media, such as for example, an oil or gas quench. If necessary, a cryogenic quench may be utilized to avoid or advantageously reduce the content of the retained austenite. By a rapid quench, the nose of the TTT curve can be avoided and the transformation to martensite can be complete. If a rapid quench does not sufficiently minimize retained austenite, the retained austenite can be reduced from the steel by a series of heat treatments at temperatures well below the γ transition temperature. Multiple ages at temperatures in the range of 1000° F. for M50 and M50NiL steels and 300-450° F. for 52100 steels will promote the formation of carbide precipitates in any retained austenite. Because carbon is an austenite (γ) stabilizer, the formation of these precipitates decreases the concentration of carbon in the austenite and promotes the conversion of austenite to a ferrite+graphite structure such as martensite.

M50NiL steel bearing materials may be carburized to a suitable case depth prior to above-described heat treatment.

Steel bearing components according to the present teachings preferably should be hardened and tempered to provide a martensitic microstructure having a hardness in the HRC58-64 (e.g., HRC60-64) range and retained austenite of less than 3% (more preferably less than 2%) by volume prior to scouring and nitriding. M50NiL steel bearing materials may have a retained austenite volume of less than 6%.

The present teachings then involve subjecting the steel bearing component to a scouring treatment (peening) or alternate treatment to impart residual compressive stresses and then to a nitriding treatment in order to achieve similar goals in improved compressive residual stresses in the component, wherein the nitriding treatment also adds the benefit of increased surface hardness.

The scouring (peening) operation may be performed on the components (e.g., bearing balls) during the grinding operations and prior to nitriding. With respect to bearing balls, the scouring operation may occur prior to hard grind for larger balls (over 1.25″ diameter) or after hard grind for smaller balls (less than 1.25″ diameter).

The scouring process may be relatively simple and may be performed using a tumbling barrel with paddles secured internally. These paddles are fixed in position and size. The rotational speed is fixed and the tumbling barrel may rotate in the clockwise direction. The number of balls disposed in the tumbling barrel at one time and the amount of time that the scouring operation is performed may be suitably adjusted to achieve a desired compressive residual stress profile prior to nitriding, such as the compressive residual stress profile for the “Tumbled” ball shown in FIG. 1, as will be discussed below. Generally speaking, and without limitation, tumbling speeds of 5-25 rpm and tumbling times of 10-60 minutes may be sufficient, depending upon, e.g., the particular steel bearing material utilized, the ball size, the number of balls, etc., to achieve a desired compressive residual stress profile prior to nitriding. Preferably, the tumbling is performed until the balls exhibit a compressive residual stress of at least −60 ksi at the surface of the ball.

However, if the bearing component is not a rolling element, in the alternate, a peening process may be performed, e.g., shot peening, according to known peening techniques, or other suitable processes utilized to generate compressive residual stresses such as low plasticity burnishing, laser shock peening, etc.

Once the steel bearing component has been scoured (peened) and ground (or vice versa), the nitriding process may be subsequently performed. Nitriding is a thermal process performed to introduce nitrogen into the surface of the component in order to impart higher hardness and associated compressive residual stresses to the steel bearing component. The nitriding process involves introduction of nitrogen (typically, in the form of ammonia, but also as nitrogen gas) into a furnace atmosphere, in which the steel component(s) has (have) been placed. The ammonia is dissociated at the surface of the component, thereby creating free nitrogen that diffuses into the component surface. The depth of the nitrided layer is temperature and time dependant. Nitriding can be performed, e.g., using either gas atmosphere with ammonia or a plasma process with nitrogen.

Additional teachings for performing the nitriding treatment are disclosed in U.S. Pat. No. 6,966,954, which is incorporated entirely herein by reference.

Once the nitriding operation has been completed, the balls may be finished using typical processing techniques in the art, such as a final lapping operation.

As a representative example of the present teachings, M50 ball samples were processed, as was generally described above, using both the mechanical scouring process and subsequent thermal nitriding step. The combination of these two processing techniques resulted in an improvement of both surface hardness due to the nitriding process and increased values of the compressive residual stress.

In particular, a surface hardness of 800-1100HV300 and a nitride depth of 0.006-0.010 inches was achieved by nitriding at temperatures from 842° F. to 1022° F. (450° C. to 550° C.) using ammonia as the nitrogen source, resulting in a nitrided case depth of approximately 0.008 inches (0.20 mm). Compressive residual stress values greater than −120 ksi (−827 MPa) to a depth of 0.002 inches (0.05 mm) and −40 ksi (−276 MPa) to a minimum depth of 0.010″ (0.25 mm) were achieved.

Compressive residual stress measurements were performed using x-ray diffraction on a TEC model 1600 or 4000 X-Ray Diffraction System.

Hardness of HRC 64-72 were observed at the surface of the ball after final processing. The hardness at the surface decays with increasing depths from the surface. A minimum nitrided case depth of 0.004 inches, more preferably 0.006 inches, measured at 800HV is preferable.

The process according to the present teachings results in a significant and surprising increase in the volume of compressive residual stress on the component while maintaining the benefit of the higher hardness from the nitriding process, as will be demonstrated with reference to the sole FIGURE.

The comparison stress curves of the FIGURE were prepared as follows:

Standard=standard processed balls. No additional processing was performed on these balls prior to testing.

Tumbled=balls that were tumbled/scoured as part of the processing prior to finishing. This scouring occurred after hard grind and prior to the final lapping operations. These balls were not nitrided.

Nitrided=Nitrided balls. These balls were nitrided after hard grind and prior to final lapping operations. However, these balls were not scoured/tumbled.

Scoured/Nitrided: These balls were treated in accordance with the present teachings, i.e. they were scoured using the same operations and sequence as the Tumbled balls plus they also received the same nitriding process as the Nitrided balls after scouring and prior to final lapping operations.

As is readily apparent from the results shown in the FIGURE, the Scoured/Nitrided balls exhibited a far greater compressive residual stress below the surface (note in particular, the range between 0.001-0.002 inches below the surface) than a combination of the tumbling and nitriding would have theoretically predicted, in view of the much smaller compressive residual stress values respectively exhibited by the Tumbled balls and the Nitrided balls.

Thus, the scouring and nitriding processes appear to act synergistically in an unexpected manner to achieve much more favorable compressive residual stress profiles.

The balls were all run on a single ball test rig for 100 hrs. The single ball test rig was operated by using a ring above and a ring below the ball to be tested. One ring is driven to cause the ball to rotate under load. Temperature-controlled oil was supplied to the ball along with the load. This test was run for 100 hrs or until failure. All tested Scoured/Nitrided balls ran until suspension of the test (i.e. 100 hrs) without failure.

As a result of this testing, improved fatigue life was demonstrated for the balls that were both scoured and nitrided as compared to the balls that did not receive both of these treatments.

Additional embodiments of the present teachings disclosed herein include, but are not limited to:

1. A method for treating a steel bearing material, comprising:

-   -   subjecting the steel bearing material to a scouring (peening)         treatment to impart compressive residual stress to a surface         region thereof; and     -   subsequently nitriding the steel bearing material.

2. The method according to embodiment 1, wherein the steel bearing component is nitrided to a depth of about 0.002-0.014 inches, preferably about 0.006-0.010 inches, more preferably about 0.008 inches.

3. The method according to embodiment 1 or 2, wherein the nitriding step is performed at one or more temperatures between 842° F. and 968° F. (450° C.-520° C.).

4. The method according to any preceding embodiment, wherein the nitriding step is performed until a compressive residual stress value greater than 100 ksi (−689 MPa), more preferably greater than −120 ksi (−827 MPa), even preferably greater than −140 ksi, and even more preferably greater than −160 ksi, is achieved at a depth of 0.002 inches (0.05 mm) and −20 ksi (−138 MPa), more preferably −40 ksi (−276 MPa), to a minimum depth of 0.010″ (0.25 mm).

5. The method according to any preceding embodiment, wherein the steel bearing component has a hardness at its surface between HRC 64-72.

6. The method according to any preceding embodiment, wherein the steel bearing component is a race and/or a rolling element.

7. The method according to embodiment 6, wherein the steel bearing component is a ball.

8. The method according to embodiment 7, wherein the scouring step is performed by:

-   -   placing the ball in a tumbling barrel having internally-secured         paddles and     -   rotating the tumbling barrel.

9. The method according to any preceding embodiment, wherein, prior to the scouring step, the steel bearing component is hardened and tempered to provide a martensitic microstructure having a hardness in the range of HRC58-64, more preferably HRC60-64, and retained austenite of less than 3% by volume.

10. The method according to any preceding embodiment, wherein the steel bearing component is comprised of, in weight percent, about 0.80-0.85% carbon, about 4.00-4.25% chromium, about 4.00-4.50% molybdenum, about 0.15-0.35% manganese, about 0.10-0.25% silicon, about 0.9-1.10% vanadium, 0.015% max. phosphorus, 0.010% max. sulfur, 0.15 max. nickel, 0.25% max. cobalt, 0.25% max. tungsten, 0.10 max. copper and the balance being essentially iron.

11. The method according to any one of embodiments 1-9, wherein the steel bearing component is comprised of, in weight percent, about 0.11-0.15% carbon, about 4.00-4.25% chromium, about 4.00-4.50% molybdenum, about 0.15-0.35% manganese, about 0.10-0.25% silicon, about 3.20-3.60% nickel, about 1.13-1.33% vanadium, 0.015% max. phosphorus, 0.010% max. sulfur, 0.25% max cobalt, 0.25% max. tungsten, 0.10 max. copper and the balance being essentially iron.

12. The method according to any one of embodiments 1-9, wherein the steel bearing component is comprised of, in weight percent, about 0.93-1.05% carbon, about 1.35-1.60% chromium, about 0.010% max molybdenum, about 0.25-0.45% manganese, about 0.15-0.35% silicon, about 0.25% max nickel, about 0.025% max. phosphorus, 0.015% max. sulfur, 0.050% max aluminum, 0.0015% max. oxygen, 0.30% max. copper and the balance being essentially iron.

13. A steel bearing component, exhibiting:

-   -   a nitrided case depth of about 0.002-0.014 inches, preferably         about 0.006-0.010 inches, more preferably about 0.008 inches;         and     -   a compressive residual stress value greater than −100 ksi (−689         MPa), preferably greater than −120 ksi (−827 MPa), more         preferably greater than −140 ksi, even more preferably greater         than −160 ksi, is achieved at a depth of 0.002 inches (0.05 mm)         and −20 ksi (−138 MPa), preferably greater than −40 ksi (−276         MPa), to a minimum depth of 0.010″ (0.25 mm).

14. The steel bearing component according to embodiment 13, wherein the steel bearing component exhibits a hardness at its surface between HRC 64-72.

15. The steel bearing component according to embodiment 13 or 14, wherein the steel bearing component is a race and/or a rolling element.

16. The steel bearing component according to embodiment 15, wherein the steel bearing component is a ball.

17. The steel bearing component according to any one of embodiments 13-16, wherein the steel bearing component primarily has a martensitic microstructure and retained austenite of less than 3% by volume.

18. The steel bearing component according to any one of embodiments 13-17, wherein the steel bearing component is comprised of, in weight percent, about 0.80-0.85% carbon, about 4.00-4.25% chromium, about 4.00-4.50% molybdenum, about 0.15-0.35% manganese, about 0.10-0.25% silicon, about 0.9-1.10% vanadium, 0.015% max. phosphorus, 0.010% max. sulfur, 0.15 max. nickel, 0.25% max. cobalt, 0.25% max. tungsten, 0.10 max. copper and the balance being essentially iron.

19. The steel bearing component according to any one of embodiments 13-17, wherein the steel bearing component is comprised of, in weight percent, about 0.11-0.15% carbon, about 4.00-4.25% chromium, about 4.00-4.50% molybdenum, about 0.15-0.35% manganese, about 0.10-0.25% silicon, about 3.20-3.60% nickel, about 1.13-1.33% vanadium, 0.015% max. phosphorus, 0.010% max. sulfur, 0.25% max cobalt, 0.25% max. tungsten, 0.10 max. copper and the balance being essentially iron.

20. The steel bearing component according to any one of embodiments 13-17, wherein the steel bearing component is comprised of, in weight percent, about 0.93-1.05% carbon, about 1.35-1.60% chromium, about 0.010% max molybdenum, about 0.25-0.45% manganese, about 0.15-0.35% silicon, about 0.25% max nickel, about 0.025% max. phosphorus, 0.015% max. sulfur, 0.050% max aluminum, 0.0015% max. oxygen, 0.30% max. copper and the balance being essentially iron.

Representative, non-limiting examples of the present invention were described above in detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Furthermore, each of the additional features and teachings disclosed above may be utilized separately or in conjunction with other features and teachings to provide improved steel bearing components, as well as methods for manufacturing and using the same.

Moreover, combinations of features and steps disclosed in the above detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention. Furthermore, various features of the above-described representative examples, as well as the various independent and dependent claims below, may be combined in ways that are not specifically and explicitly enumerated in order to provide additional useful embodiments of the present teachings.

All features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter, independent of the compositions of the features in the embodiments and/or the claims. In addition, all value ranges or indications of groups of entities are intended to disclose every possible intermediate value or intermediate entity for the purpose of original written disclosure, as well as for the purpose of restricting the claimed subject matter. 

We claim:
 1. A method for treating a steel bearing component, comprising: subjecting the steel bearing component to a scouring treatment to impart compressive residual stress to a surface region thereof; and subsequently nitriding the steel bearing component.
 2. The method according to claim 1, wherein the steel bearing component is nitrided to a depth of about 0.002-0.014 inches.
 3. The method according to claim 1, wherein the steel bearing component is nitrided to a depth of 0.006-0.010 inches.
 4. The method according to claim 1, wherein the nitriding step is performed at one or more temperatures between 842° F. and 968° F.
 5. The method according to claim 1, wherein the nitriding step is performed until a compressive residual stress value greater than −120 ksi is achieved at a depth of 0.002 inches and −40 ksi to a minimum depth of 0.010″.
 6. The method according to claim 1, wherein the nitriding step is performed until a compressive residual stress value greater than −160 ksi is achieved at a depth of 0.002 inches.
 7. The method according to claim 1, wherein the steel bearing component has a hardness at its surface between HRC 64-72 after the nitriding step.
 8. The method according to claim 1, wherein the steel bearing component is a race and/or a rolling element.
 9. The method according to claim 8, wherein the steel bearing component is a ball.
 10. The method according to claim 1, wherein the scouring step is performed by: placing the ball in a tumbling barrel having internally-secured paddles and rotating the tumbling barrel.
 11. The method according to claim 1, wherein, prior to the scouring step, the steel bearing component is hardened and tempered to provide a martensitic microstructure having a hardness in the range of HRC60-64 and retained austenite of less than 3% by volume.
 12. The method according to claim 1, wherein the steel bearing component is comprised of, in weight percent, about 0.80-0.85% carbon, about 4.00-4.25% chromium, about 4.00-4.50% molybdenum, about 0.15-0.35% manganese, about 0.10-0.25% silicon, about 0.9-1.10% vanadium, 0.015% max. phosphorus, 0.010% max. sulfur, 0.15 max. nickel, 0.25% max. cobalt, 0.25% max. tungsten, 0.10 max. copper and the balance being essentially iron.
 13. The method according to claim 1, wherein the steel bearing component is comprised of, in weight percent, about 0.11-0.15% carbon, about 4.00-4.25% chromium, about 4.00-4.50% molybdenum, about 0.15-0.35% manganese, about 0.10-0.25% silicon, about 3.20-3.60% nickel, about 1.13-1.33% vanadium, 0.015% max. phosphorus, 0.010% max. sulfur, 0.25% max cobalt, 0.25% max. tungsten, 0.10 max. copper and the balance being essentially iron.
 14. The method according to claim 3, wherein the nitriding step is performed at one or more temperatures between 842° F. and 968° F. and until a compressive residual stress value greater than −160 ksi is achieved at a depth of 0.002 inches; and wherein the steel bearing component has a hardness at its surface between HRC 64-72 after the nitriding step.
 15. The method according to claim 14, wherein the steel bearing component is a ball; and the scouring step is performed by: placing the ball in a tumbling barrel having internally-secured paddles and rotating the tumbling barrel.
 16. The method according to claim 15, wherein, prior to the scouring step, the ball is hardened and tempered to provide a martensitic microstructure having a hardness in the range of HRC60-64 and retained austenite of less than 3% by volume.
 17. A steel bearing component, exhibiting: a nitrided case depth of about 0.002-0.014 inches; and a compressive residual stress value greater than −120 ksi at a depth of 0.002 inches.
 18. The steel bearing component according to claim 17, wherein: the nitrided case depth is 0.006-0.010 inches; the compressive residual stress value is greater than −160 ksi at a depth of 0.002 inches and −40 ksi to a minimum depth of 0.010″, and the steel bearing component exhibits a hardness at its surface between HRC 64-72.
 19. The steel bearing component according to claim 18, wherein the steel bearing component is a ball, primarily has a martensitic microstructure and has a retained austenite content of less than 3% by volume.
 20. The steel bearing component according to claim 19, wherein the ball is comprised of, in weight percent: about 0.80-0.85% carbon, about 4.00-4.25% chromium, about 4.00-4.50% molybdenum, about 0.15-0.35% manganese, about 0.10-0.25% silicon, about 0.9-1.10% vanadium, 0.015% max. phosphorus, 0.010% max. sulfur, 0.15 max. nickel, 0.25% max. cobalt, 0.25% max. tungsten, 0.10 max. copper and the balance being essentially iron, or about 0.11-0.15% carbon, about 4.00-4.25% chromium, about 4.00-4.50% molybdenum, about 0.15-0.35% manganese, about 0.10-0.25% silicon, about 3.20-3.60% nickel, about 1.13-1.33% vanadium, 0.015% max. phosphorus, 0.010% max. sulfur, 0.25% max cobalt, 0.25% max. tungsten, 0.10 max. copper and the balance being essentially iron. 